L(24) is a membrane-spanning peptide of the structure KKL(24)KK and LA(12) has a structure KK(LA)(12)KK. We investigated effects of these peptides on alkyl chain order, alkyl chain motional freedom, polarity of the membrane interior, and on oxygen transport. As a model membrane we used liposomes and oriented membranes. We used PC-spin labels with nitroxide moiety located at the 5-, 7-, 10-, 12-, and 16- position. We showed that L(24) ordered the lipid bilayer, decreased motional freedom of alkyl chains and significantly increased the hydrophobicity of the membrane interior. The effects of LA(12) on these membrane properties were very weak.